1. Field of the Invention
The present invention relates to a switching regulator that converts the level of a direct-current voltage by turning on and off a switching device.
2. Description of the Prior Art
First, a prior-art example will be described with reference to FIG. 4, which shows a conventional switching regulator of a step-down type. A p-channel MOS field-effect transistor (hereinafter referred to simply as a xe2x80x9ctransistorxe2x80x9d) Tr is used as a switching device, and the source of this transistor Tr is connected to an input terminal IN. The drain of the transistor Tr is connected to the cathode of a diode D and also to one end of a coil L. The anode of the diode D is grounded. The other end of the coil L is grounded through a smoothing capacitor C. The node between the coil L and the capacitor C is connected to a terminal OUT.
The voltage th at appears at the terminal OUT for output is divided by resistors R1 and R2, and the thus divided voltage Vd is, together with a reference voltage Vref, fed to an error amplifier EA, which outputs a voltage according to the difference between the two voltages fed thereto. Specifically, as the voltage Vd increases relative to the reference voltage Vref, the output voltage of the error amplifier EA decreases, and, as the voltage Vd decreases relative to the reference voltage Vref, the output voltage of the error amplifier EA increases.
The output voltage of the error amplifier EA is, together with a triangular-wave voltage output from a triangular wave generating circuit TPG, fed to a comparator COMP, which outputs a high-level voltage when the voltage of the triangular wave is higher than the output voltage of the error amplifier EA and outputs a low-level voltage when the voltage of the triangular wave is lower than the output voltage of the error amplifier EA. The voltage output from the comparator COMP is fed through a buffer amplifier BA to the gate of the transistor Tr.
The circuit configuration described above keeps the transistor Tr turned on and off repeatedly. When the transistor Tr is turned from on to off, a back electromotive force appearing in the coil L tends to keep a current flowing from ground through the diode D. This causes the drain voltage of the transistor Tr to become lower than the ground voltage by the voltage drop across the diode D, and thus the direct-current voltage fed out via the terminal OUT (this voltage will hereinafter be referred to as the xe2x80x9coutput voltagexe2x80x9d) becomes lower than the direct-current voltage fed in via the terminal IN.
The level of the output voltage varies according to the duty ratio of the xe2x80x9conxe2x80x9d periods of the transistor Tr (specifically, the greater the duty ratio, the higher the output voltage). When the level of the output voltage is higher than a predetermined level, the duty ratio decreases, and, when the level of the output voltage is lower than the predetermined level, the duty ratio increases. In this way, the level of the output voltage is stabilized at the predetermined level.
However, in this conventional switching regulator, as shown in FIG. 5, the signal OCOMP output from the comparator COMP is a pulse signal that abruptly rises and drops (in other words, a signal that contains higher-frequency, or harmonic, components), and this signal is directly fed to the gate of the transistor Tr to turn it on and off. As a result, quite inconveniently, high-frequency noise N of the order of tens of millivolts appears in the output voltage VOUT. For this reason, conventional switching regulators cannot be used in audio and visual applications, and it has customarily been inevitable to use instead series regulators that are inefficient and generate much heat.
An object of the present invention is to provide a switching regulator that outputs a direct-current voltage with significantly reduced high-frequency noise.
To achieve the above object, according to the present invention, a switching regulator that converts the level of a direct-current voltage by turning a switching device on and off is provided with a waveform shaping circuit that blunts the waveform of the signal with which the switching device is turned on and off. With this circuit configuration, it is possible to remove high-frequency components from the signal with which the switching device is turned on and off.